Thermionic cathodes



April 3, 1956 J. N. BOWTELL ETA!- THERMIONIC CATHODES Filed July 14, 1952 Inventors:

Jam es Norman Bowt ell Edward Em st Miles, 0225 orney Their THERMIONIC CATHODES James Norman Bowtell and Edward Ernest Miles,

Wembley, England, assignors to General Electric Company, a corporation of New York Application July 14, 1952, Serial No. 298,849 Claims priority, application Great Britain July 16, 1951 3 Claims. or. 313-109) a This invention relates to thermionic cathodes for electric discharge devices having a gas or vapor filling, the

electron emission and to be resistant to ion bombardment, but these properties, which dependon the electron emissive material used, do not in general go together. Thus, for example, alkaline earth metal oxide material, which is most commonly used, is more active than, say, barium silicate but is not so resistant to ion bombardment.

The object of this invention is to provide a cathode of the kind referred to which provides a good electron emission and is, at the same time, resistant to ion bombardment.

According to the invention, in a thermionic cathode of the kind referred to the electron emission material comprises a mixture of at least one alkaline earth metal oxide with at least one alkaline earth metal tungstate of the form XsWOs, where X is alkaline earth metal, W is tungsten and O is oxygen.

' It is to lac-understood that the term alkaline earth metal as used herein refers to the metals calcium, strontium and barium.

The single figure of the drawing shows a fluorescent lamp embodying the invention.

In cathodes for use in low pressure fluorescent electric discharge lamps, the electron emissive material preferably contains only a minor proportion of the alkaline earth metal tungstate, the greater part of the material consisting of one or more alkaline earth metal oxides. In such cathodes, the proportion of tungsten present as the tungstate is preferably within the range of about 1% to of the total weight of the alkaline earth metal oxide, the optimum proportion of tungsten within this range being about 4%. The alkaline earth metal tungstate present preferably consists of a barium strontium tungstate of the form BazSrWOs.

In the case of cathodes in which the electron emissive material consists of a coating on the metal carrier, and especially when the carrier is a coiled-coil helix of the known kind formed by coiling an already coiled wire, we have found that the best results are obtained by forming the alkaline earth metal tungstate in situ on the carrier.

This is preferably effected by mixing finely powdered tungsten or tungsten oxide in the alkaline earth metal carbonate suspension usually used and then applying the suspension to the metal carrier, drying the coating thus formed, and heating the cathode, in the usual manner.

The suspending medium for the mixture of carbonate ICC 2 or carbonates and powdered tungsten or tungsten oxide may, for example, be a solution of nitrocellulose in butyl acetate. The particle size of the tungsten or tungsten oxide powder may conveniently lie in the range of about /2 toS microns and is preferably about one micron. The carbonate constituent preferably consists of a mixture of barium and strontium carbonates in equimolecular proportions. in cathodes for use in low pressure fluorescent electric discharge lamps, the proportion of tungsten included in the mixture in the form of the powdered metal or oxide is suitably within the range of about 0.7% to 7.5% by weight of the total carbonates, the preferred proportion being about 3% by weight of the carbonates.

The coated and dried cathode is mounted in the electric. discharge device in known manner, and the cathode is subsequently heated during the processing of the device to a temperature of about 1300 0., usually by the passage of an electric current through the metal carrier. During this heating the carbonates are reduced to oxides; at the same time a reaction of part of the carbonates or oxides or both with the tungsten or tungsten oxide takes place to form the alkaline earth metal tungstate or tungstates in admixture with alkaline earth metal oxide, and a lightly sintered cathode coating is obtained. In some cases, especially with the higher proportions of tungsten, some free tungsten or tungsten oxide may be left, but its presence is not usually disadvantageous.

Referring to the drawing, the invention may be embodied in a fluorescent lamp of well known kind comprising a vitreous envelope 1 coated internally with a phosphor and containing a conventional filling of mercury and an inert starting gas at a low pressure. The electrodes 2, at opposite ends of the envelope, are supported on lead wires 3 sealed through the ends of the lamp. Each electrode consists of a coiled coil tungsten filament serving as a carrier coated with a lightly sintered coating 4 composed of oxides of barium and strontium and a minor proportion of barium strontium tungstate of the form BaaSrWOs, in the preferred proportions given earlier.

We have found that low pressure fluorescent electric discharge lamps provided with cathodes in accordance with the invention, especially of the filamentary coiledcoil type, and containing the usual filling of mercury and inert ionizable gas, give reliable starting and have exceptionally long life with good lumen maintenance. With a cathode in the form of a coated metal filament in which thetungstate constituent is formed in situ, the coating is firmly attached to the filament, which facilitates handling and transport of the lamp.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A low pressure fluorescent lamp comprising a sealed light-transmitting envelope having a coating of fluorescent material on the inner wall thereof, a quantity of mercury and a filling of inert gas in said envelope, coiled filamentary electrodes .in said envelope, and a lightly sintered coating on said electrodes composed of oxide of alkaline earth metal and a minor proportion of alkaline earth metal tungstate of the form XsWOe such that the weight of tungsten present in the tungstate is in the range of about 1% to 10% of the total weight of the alkaline earth metal oxide, where X is alkaline earth metal, W is tungsten and O is oxygen.

2. A low pressure fluorescent lamp comprising a sealed light-transmitting envelope having a coating of fiurescent material on the inner wall thereof, a quantity of mercury and a filling of inert gas in said envelope, coiled filamentary electrodes in said envelope, and a lightly sintered coating on said electrodes composed of oxide of alkaline earth metal and a minor proportion of alkaline earth metal tungstate of the form XgWOc such that the weight of tungsten present in the tungstate is about 4% of the 4 total weight of the alkaline earth oxide, where X is tungstate of the form BaaSrWOe such that the weight of alkaline earth metal, W is tungsten and O is oxygen. I tungsten present in the tungstate is about 4 per cent of 3. A low pressure fluorescent lamp comprising a sealed the total weight of barium and strontium in the coating. light transmitting envelope having a coating of fluorescent material on the inner wall thereof, a quantity of mercury 5 R f e Cited in the file of this patent and a filling 0r inert gas 1n said envelope, coilen fila- UNITED STATES PATENTS mentary electrodes 1n said envelope, and a lightly s1ntered coating on said electrodes composed of oxide of barium 2,175,345 Galdles Oct. 10, 1939 and strontium and a minor proportion of barium strontium 

1. A LOW PRESSURE FLUORESCENT LAMP COMPRISING A SEALED LIGHT-TRANSMITTING ENVELOPE HAVING A COATING OF FLUORESCENT MATERIAL ON THE INNER WALL THEREOF, A QUANTITY OF MERCURY AND A FILLING OF INERT GAS IN SAID ENVELOPE, COILED FILAMENTARY ELECTRODES IN SAID ENVELOPE, AND A LIGHTLY SINTERED COATING ON SAID ELECTRODES COMPOSED OF OXIDE OF ALKALINE EARTH METAL AND A MINOR PROPORTION OF ALKALINE EARTH METAL TUNGSTATE OF THE FORM X3WO6 SUCH THAT THE WEIGHT OF TUNGSTEN PRESENT IN THE TUNGSTATE IS IN THE RANGE OF ABOUT 1% TO 10% OF THE TOTAL WEIGHT OF THE ALKALINE EARTH METAL OXIDE, WHERE X IS ALKALINE EARTH METAL, W IS TUNGSTEN AND O IS OXYGEN. 